We propose to continue our studies of the extent to which cell cycle properties of normal rapidly proliferating tissues can be used to modulate tissue toxicity during treatment with cytosine arabinoside (Ara-C), a cell cycle phase-specific agent which exhibits clinically dose-limiting gastrointestinal and hematopoietic toxicity. Understanding the principles behind normal tissue toxicity should ultimately allow development of rational kinetically-based schedules to modulate normal tissue damage induced not only by Ara-C, but by other treatment protocols as well. Our plan is to obtain cytokinetic information on stem-cell populations in murine intestine and bone marrow following 1, 2, 3, and 6 doses of Ara-C to allow us to understand and subsequently modulate damage to these populations in a ten dose treatment schedule. First, to facilitate analysis of stem-cell cytokinetic response, we wll apply dual beam flow cytometry and sorting, multivariate analysis and indexed sorting to discriminate and/or enrich gastrointestinal and hematopoietic (CFU-S and CFU-GM) stem-cell populations. Secondly, we will estimate cell cycle traverse rates, recruitment and cell loss rate in the stem-cell populations during and after Ara-C treatment to provide information on the number of remaining clonogenic S-phase cells and total stem cells. These data will be confirmed by clonogenic stem cell assays. Thirdly, we will use extended Ara-C infusion periods to characterize the cytokinetic nature of cells which survive aggressive Ara-C therapy. Finally, these measurements will be evaluated collectively in a mathematical model to predict the extent of stem cell damage occurring as a function of dose interval in a ten dose treatment schedule. Experimental measurements of stem cell survival will then be compared with the model prediction. The degree of similarity between the predicted values and the experimental observations will indicate the extent to which cytokinetic data can be used to minimize destruction of a particular subpopulation in dose-limiting normal tissue. Quantitation of normal tissue toxicity produced by well-timed multiple dose schedules and by continuous infusion will be included with similar data on a human tumor in nude mice to determine the practicality of using cytokinetic information to design effective chemotherapeutic protocols with acceptable therapeutic ratios.